<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <script src="js/vue.js"></script>
    <title>Document</title>
</head>
<body>
    <div id="app"></div>
</body>
<script>
    var vm = new Vue({
        el:'#app',
        data:{
            firstName:'Foo',
            lastName:'Bar',
            //fullName:'Foo Bar'
        },
        watch:{
            firstName:function(val){
                this.fullName=val+' '+val;
            }
        },
        computed:{
            fullName:{
                get:function(){
                    return this.firstName+' '+this.lastName;
                },
                set:function(newValue){
                    var names=newValue.split(' ');
                    this.firstName=names[0];
                    this.lastName=names[names.length-1];
                }
            }
        }
    })
</script>
</html>